1. Field of the Invention
The present invention relates to a heat-sensitive recording material, and particularly, to a heat-sensitive recording material which has superior device suitability with no reduction in device endurance, such as head abrasion resistance, has image suitability and stability as a recording medium, is suitable for printing, stamping or sealing, and with respect to environment, and is useful as a recording medium having a high adhesive strength when an adhesive sheet such as a revenue stamp, a postage stamp or a tack sheet is adhered thereto.
2. Description of the Related Art
In the 1960s, NCR Co. historically developed, as a heat-sensitive recording material technology, a dye-type heat-sensitive sheet using a colorless leuco dye and a phenolic acidic material. This type of heat sensitive sheet represents the main stream of heat-sensitive recording material systems. Since then, the demand for heat-sensitive sheets (referred to as “heat-sensitive recording materials” hereinafter) has grown drastically on the basis of the following: (1) a thermal head was developed against the background of semiconductor technology and the device became small in size and inexpensive due to a remarkable improvements in cost and performance thereof, (2) the quality of the heat-sensitive sheet (heat-sensitive recording material) itself was improved (improvement in sensitivity and head-matching, and so on), along with item (1), and (3) when the heat-sensitive recording material system was compared with various recording systems, such as electrostatic recording, inkjet recording, and PPC recording systems, it was evaluated to be more convenient, i.e., simple, inexpensive, maintenance-free, and so on.
However, as heat-sensitive recording materials have been used in facsimiles, various printers, or the like on the basis of the conveniences thereof and have become familiar in daily life, drawbacks of the heat-sensitive recording materials have become well-known. That is, the following drawbacks have been pointed out:                The heat-sensitive recording materials become discolored by light.        The heat-sensitive recording materials become discolored when stored at high temperature (for example, when left in a car).        Colors of recorded images fade away when exposed to chemical agents (such as plasticizers in wrap film, oils and fats, organic solvents used in markers, and ink for an inkjet printers).        The heat-sensitive recording materials do not feel like plain paper, which, for example, 1) has good stamping or sealing suitability (stamped ink is satisfactorily dries and does not bleed), 2) has a recording surface that is a mat surface and is easy and comfortable to read, 3) is able to have characters or the like be satisfactorily written thereon with pencil or the like, and 4) does not easily generate blemishes due to rubbing.        
Accordingly, there has been demand to overcome the above-mentioned drawbacks in the conventional heat-sensitive recording materials and to develop and provide heat-sensitive recording materials whose added values are high.
In recent years, heat-sensitive recording materials have been widely used since they are generally relatively inexpensive and since recording devices for use therewith are compact and maintenance-free. Under such conditions, competition with respect to sales of heat-sensitive recording materials has recently intensified. Thus, heat-sensitive recording materials having higher, non-conventional functions, and particularly, heat-sensitive recording materials which can provide improved color-developed density and vividness, have superior background whiteness and storage stability, and can provide good and vivid hue for a full color image recorded by inkjet printing or the like have been demanded. To meet these demands, various qualities of heat-sensitive recording materials, such as color-developability and storability, have eagerly been investigated.
Examples of qualities that heat-sensitive recording materials should have include 1) high sensitivity (a high density can be obtained), 2) high background portion (non-image portion) whiteness, (a low level of fog in background portions) 3) superiority storability of an image after the image is printed, 4) superior light resistance, 5) superior chemical resistance, 6) vividness and high image quality, 7) capability for a variety of good and vivid hues for a full color image recorded by inkjet printing, 8) good head-matching to thermal heads and good suitability for thermal heads (hardly any abrasion to thermal heads is hardly caused), 9) a similar feel as plain paper, 10) good printing suitability such that printing can be achieved on a heat-sensitive color-developable layer, and good stamping or sealing suitability such that bleeding does not occur, 11) good adaptability to high-performance printers such as high-speed printers, and 12) friendliness to the environment. Under the above-mentioned circumstances, there is demand for heat-sensitive recording materials to have all of these qualities simultaneously without lacking in any one of them. However, at present, heat-sensitive recording materials simultaneously having the above-mentioned qualities have not yet been provided.
Hitherto, 2,2-bis(4-hydroxyphenyl)propane (so-called bisphenol A) has been widely used as an electron-accepting compound which reacts with an electron-donating colorless dye used in heat-sensitive recording materials to cause color-development. However, background fog and image storability have not yet been satisfactory according to this system.
The system also has a problem with respect to suitability for inkjet printing, as described below. When full color information is recorded on a heat-sensitive recording material, the recording may be performed using ink for inkjet printing. In this case, when an ordinary heat-sensitive recording material using bisphenol A is utilized to perform inkjet printing on a recording face thereof, a hue of the ink cannot be faithfully reproduced, and the hue cannot be made vivid. Alternatively, color of a recorded image that has previously been formed on the heat-sensitive recording material may fade away. Also, when the above-described ordinary heat-sensitive recording material is put on a medium on which an image is recorded by means of an inkjet printer in the state of contact therebetween, background fog may be generated or the color of the recorded image may fade away.
Recently, heat-sensitive sheets have been very frequently used as sheets for cash register printers. The sheets for cash registers may be used, as is, as receipts. Thus, there is increasing demanded for development of a heat-sensitive sheet from which an affixed revenue stamp is not easily peeled off. Since a postage stamp would also not be easily peeled off from such a heat-sensitive recording sheet, it is conceivable that the heat-sensitive recording sheet could be applied to postcards or the like. However, when compared with ordinary paper, conventional heat-sensitive sheets have weaker adhesiveness with respect to revenue stamps or postage stamps, and, under the present circumstances, even if such a stamp is affixed thereto, the stamp is easily peeled off.